bmcweb: map PCIe Gen6 generation

Add Gen6 to redfishPcieGenerationFromDbus so a Gen6 device reports
PCIeType Gen6 instead of failing as an unknown generation. The Gen6
value was added to the PCIeSlot generations in phosphor-dbus-interfaces:
https://github.com/openbmc/phosphor-dbus-interfaces/commit/8d09e7d

Tested: Built for nvl32-obmc; a Gen6 GPU now reports PCIeType "Gen6".

Change-Id: I8b51d407887e57a3479b8a39cbbe08aa2993e8b2
Signed-off-by: Ender Hsieh <andhsieh@nvidia.com>